The present invention relates to scintillator crystals, to a manufacturing process allowing them to be obtained and to the use of the said crystals, especially in gamma-ray and/or X-ray detectors.
A scintillator crystal is a crystal which is transparent in the scintillation wavelength range which responds to incident radiation by emitting a light pulse. Scintillator crystals are widely used in detectors for gamma-ray, X-rays, cosmic rays and particles whose energy is of the order of 1 keV and greater. From such crystals it is possible to manufacture detectors in which the light emitted by the crystal that the detector comprises is coupled to a light-detection means and produces an electrical signal proportional to the number of light pulses received and to their intensity. In scintillation devices the detector is generally a single scintillator crystal.
Solid state scintillator crystals are in common use as components of radiation detectors in X-ray detection apparatus such as counters, image intensifiers and computerized tomography (CT) scanners. Such detectors are used especially in the fields of nuclear medicine, physics, chemistry and oil well logging. One embodiment of the present generation of scintillators comprises oxide mixtures in which a rare earth oxide is present as an activator, along with various combined matrix elements which are also usually rare earth oxides. Other combined metals may also be present as additives for specific purposes. These scintillators have been characterized by the advantageous properties of high efficiency, moderate decay time, low afterglow and little or no radiation damage upon exposure to high X-ray doses.
A family of known scintillator crystals widely used is of the thallium-doped sodium iodide, or NaI:Tl, type. Crystals of the NaI:Tl family have a low density and therefore a low detection efficiency for certain types of high-energy radiation; they also have hygroscopic problems.
Another family of scintillator crystals is of the barium fluoride (BaF2)type. Crystals of the BaF2 family are not very dense and their rapid emission component lies within the ultraviolet range, which means the use of expensive photodetectors in scintillation devices.
Another family of scintillator crystals which has undergone considerable development is of the bismuth germanate (BGO) type. Crystals of the BGO family have a long scintillation decay time which limits the use of such crystals to low counting rates.
A more recent family of scintillator crystals was developed in the 1980s and is of the cerium-activated gadolinium orthosilicate (GSO) type. Crystals of the GSO family have a low optical yield and a strong tendency to cleave, which makes them extremely difficult to prepare.
A new family of crystals was developed at the end of the 1980s in order to obtain scintillator crystals having a high light yield, short luminescence decay times and a high detection efficiency: these crystals are of the cerium-activated lutetium oxyorthosilicate (LSO) type and formed the subject-matter of U.S. Pat. No. 4,958,080. A method of growing such a crystal formed the subject-matter of U.S. Pat. No. 5,660,627. Although the scintillation properties of the crystals of this family are excellent, they do have a major drawback with regard to reproducibility, which has a negative impact on the development of their use. This is because the results of scintillation properties between two crystals of the same composition may vary very considerably as indicated, for example, by the following publications: xe2x80x9cCe-doped scintillators: LSO and LuAPxe2x80x9d (A. Lempicki and J. Glodo, Nuclear Instruments and Methods in Physics Research A416 (1998), 333-344) and xe2x80x9cScintillation Light Emission Studies of LSO Scintillatorsxe2x80x9d (A. Saoudi et al., IEEE Transactions on Nuclear Science, Vol. 46, No. 6, December 1999). These authors indicated in particular the difficulties of using LSO owing to very large variations in the scintillation properties of LSO single crystals from one crystal to another, even when they are cut from the same ingot.
Another drawback with LSO relates to its high melting point, about 2200xc2x0 C. and this means that the process allowing such a crystal to be obtained requires high temperatures.
The latest scintillator compositions employ at least one of the oxides of lutetium, yttrium and gadolinium as matrix materials. These are described in detail, for example, in U.S. Pat. Nos. 4,421,671, 4,473,513, 4,525,628, 4,783,596, and 6,093,347. These crystals typically comprise a major proportion of yttria (i.e., Y2O3), up to about 50 mole percent gadolinia (Gd2O3) and a minor activating proportion of a rare earth activator oxide. Suitable activator oxides, as described in the aforementioned patents, include the oxides of europium, neodymium, ytterbium, dysprosium, terbium and praseodymium. Europium-activated scintillators are often used in commercial X-ray detectors by reason of their high luminescent efficiency, and low afterglow level. Decay times of such scintillators are on the order of 0.9-1.0 millisecond.
The search thus continues for scintillator compositions having improved properties.
The object of the present invention is to alleviate these drawbacks and to propose a novel family of scintillator crystals whose scintillation properties are of the same order of magnitude as those of LSO crystals, wherein the property variations from one crystal to another of the same composition are very much less than the property variations from one LSO crystal to another of the same composition.
One crystal according to the invention is a monoclinic single crystal obtained by crystallization of a congruent molten composition of general formula:
LU2(1xe2x88x92x)M2xSi2O7
where LU is selected from lutetium, or a lutetium-based alloy which also includes one or more of the elements Sc, Y, In, La, Gd; where M is cerium, or cerium partially substituted with one or more of the elements of the lanthanide family (excluding lutetium); and where x is a variable defined by the limiting level of Lu substitution with M in a monoclinic crystal of the lutetium pyrosilicate (LPS) structure.
One crystal according to the invention is a monoclinic single crystal obtained by crystallization of a congruent molten composition of general formula:
LU2(1xe2x88x92x)M2xSi2O7
where LU is selected from lutetium, or a lutetium-based alloy which also includes one or more of the elements Sc, Y, In, La, Gd; where M is cerium, or cerium partially substituted with one or more of the elements of the lanthanide family (excluding lutetium); and where x is a variable defined by the limiting level of Lu substitution with M in a monoclinic crystal of the lutetium pyrosilicate (LPS) structure.
The invention allows of producing scintillator crystals of high quality based on lutetium alloys. Lutetium is a very expensive metal. It is possible by utilizing alloys to retain the host lattice structure for dopants that lutetium pyrosilicate offers, while considerably reducing the cost of the raw materials needed to produce the scintillator crystal. This solution is also advantageous as it makes it possible to lower the melting point of the crystal. The elements mentioned for making up the said alloy are optically inert. Typically, if a lutetium-based alloy is used, the quantity of Sc, Y, In, La, and Gd is less than about 25% by weight of the lutetium-based alloy.
It is also possible to obtain monoclinic single crystals according to the invention in which lutetium is mixed with one or more elements from Sc, Y, In, La, Gd, in order to form an alloy in which the LPS structure is retained and in which it is possible to insert cerium or cerium partially substituted with one or more of the elements from the lanthanide family other than lutetium (elements of atomic number ranging between 57 (in the case of lanthanum) and 70 (in the case of ytterbium)).
According to another variant, the crystal is of general formula:
LU2(1xe2x88x92x)M2xSi2O7
where x is defined by the limiting level of Lu substitution with M in a monoclinic crystal of the lutetium pyrosilicate (LPS) structure.
According to another variant, the crystal is of general formula:
LU2(1xe2x88x92x)Ce2xSi2O7
where x is defined by the limiting level of Lu substitution with Ce in a monoclinic crystal of the lutetium pyrosilicate (LPS) structure.
Advantageously, the level of x substitution is less than 0.5 at %, preferably less than 0.2 at % and more preferably less than 0.1 at %. In certain embodiments, the level of x substitution is greater than 0.0001 (or 0.01%) and in other embodiments the level of x is greater than 0.0005 (or 0.05%).
According to an advantageous variant, the single crystal according to the invention has at least two dimensions each greater than 3 mm, preferably greater than 5 mm, and even more preferably greater than 10 mm. The crystals of this size are advantageously obtained by growth from a congruent melt pool using the Czochralski method. The Czochralski method is known by those skilled in the art for growing single crystals.
An important embodiment of the invention is compositions wherein the single crystals produced from the same chemical composition have a relative variation in light yield under identical excitation of less than 50%. In a preferred embodiment of the invention, at least 80 percent of crystals with the same composition, prepared in the same method using the same reactants, will vary in photons/MeV response to a gamma source such as 241Am by less than 20%.
Without being theory, it may be considered that the excellent scintillation results and the good reproducibility which are observed in LPS single crystals may be due to the presence of insertion sites of a single type in the lutetium pyrosilicate structure, whereas the heterogeneity problems in the LSO crystals are especially due to the possible presence of several insertion sites for the cerium in the LSO structure.
An important embodiment of the invention is compositions which melt at a temperature of less than 2200xc2x0 C., preferably less than 2100xc2x0 C., even more preferably less than 2000xc2x0 C., for example around 1900xc2x0 C.
In another embodiment of the invention the decay time for the crystal is less than about 70 nanoseconds, preferably less than about 50 nanoseconds, more preferably less than about 30 nanoseconds, even more preferably less than about 15 nanoseconds. Scan times of scintillation devices are often related to primary decay time of the scintillator roughly by a factor of 1,000. Thus, a scintillator having a decay time of 1 millisecond will typically produce a scan time of about 1 second. The scanning units containing the present generation of scintillators have scan times on the order of 1 second. Shorter scan times are desired. Shorter scan times may be achievable if the primary decay time of the scanner is shortened. In general, scan time in seconds is associated with a primary decay time of an equal number of milliseconds.
The invention also relates to a process for manufacturing scintillator single crystals having a light yield under gamma excitation of greater than about 10,000 photons per MeV, in which the single crystals produced from the same chemical composition have a relative variation in light yield under identical excitation of less than 50%, comprising at least the following steps:
Supplying raw material powders, for example lutetium oxide (Lu2O3) powder, silica (SiO2) powder, and powder of at least one caesium carrier, for example an oxide (Ce2O3), advantageously at predetermined stoichiometric quantities;
Optionally pre-reacting the powders supplied;
Melting of the powders and/or optionally pre-reacted powders at a temperature of less than 2200xc2x0 C., preferably less than 2100xc2x0 C., and even more preferably less than 2000xc2x0 C., in order to obtain a molten composition of general formula
LU2(1xe2x88x92x)M2xSi2O7
where LU is lutetium or a lutetium-based alloy which also includes one or more of the following elements: Sc, Y, In, La, Gd; where M is cerium or cerium partially substituted with one or more of the elements of the lanthanide family (excluding lutetium); and where x is defined by the limiting level of Lu substitution with M in a monoclinic crystal of the lutetium pyrosilicate (LPS) structure;
Growing of a crystal by a floating zone technique or according to the Czochralski method; and
Cooling of the single crystal.
This process is particularly advantageous since the temperatures needed to melt and then grow the LPS crystals are markedly lower, about 200xc2x0 C. lower, than those needed for growing LSO crystals. Thus, a process is obtained which is less expensive than that needed for the manufacture of LSO single crystals. This point is particularly important since the techniques for growing this type of crystal generally require crucibles made of iridium, the melting point of which is 2410xc2x0 C., into which the raw materials for the melt pool are introduced and heated. It is known that iridium starts to soften around 2100xc2x0 C., thereby shortening the lifetime of the crucibles used at this temperature. By virtue of the melting point of LPS of about 1900xc2x0 C., it is possible to considerably reduce the investment and running costs of the crucibles, especially by reducing the thickness of the walls of the crucibles and/or by benefiting from a longer lifetime compared with that of crucibles made to operate at a temperature of around 2100xc2x0 C. or higher. Further advantages arise owing to the melting point of LPS, about 1900xc2x0 C., such as, in particular, energy savings and longer lifetimes of the refractories used in the furnaces, compared with the conditions required by a crystal melting at around 2100xc2x0 C.
The crystal may be annealed in a vacuum of at least about 30 millimeters of mercury pressure, or an atmosphere of an inert gas such as argon, or in an atmosphere of an inert gas such as argon with a reducing gas such as molecular hydrogen. While annealing benefits from extended time, the annealing process can be as short as about 30 minutes.
The invention also relates to a radiation detector, especially for gamma-rays and/or X-rays, containing: a scintillator consisting of a transparent monoclinic single crystal of general formula:
LU2(1xe2x88x92x)M2xSi2O7
where LU is lutetium or a lutetium-based alloy which also includes one or more of the following elements: Sc, Y, In, La, Gd; where M is cerium or cerium partially substituted with one or more of the elements of the lanthanide family (excluding lutetium); and where x is defined by the limiting level of LU substitution with M in a monoclinic crystal of the lutetium pyrosilicate (LPS) structure; a photodetector optically coupled to the scintillator in order to produce an electrical signal in response to the emission of a light pulse produced by the scintillator. The photodetector of the detector may especially any type known in the art, for example a photomultiplier, or a photodiode, or else a CCD sensor.
The preferred use of this type of detector relates to the measurement of gamma-rays or X-rays; such a system is also capable of detecting alpha and beta particles as well as electrons.
According to an advantageous variant, the detector comprises a scintillator of general formula
LU2(1xe2x88x92x)M2xSi2O7
where x is defined by the limiting level of Lu substitution with M in a monoclinic crystal of the lutetium pyrosilicate (LPS) structure. According to another advantageous variant, the detector comprises a scintillator of general formula
LU2(1xe2x88x92x)Ce2xSi2O7
where x is defined by the limiting level of Lu substitution with Ce in a monoclinic crystal of the lutetium pyrosilicate (LPS) structure.
According to an advantageous variant, the scintillator single crystal has a level of x substitution of less than 0.5 at %, especially less than 0.2 at %, and even less than 0.1 at %. The level of x substitution is advantageously greater than 0.01 at % in some embodiments, and more advantageously greater than about 0.05 at % in some embodiments.
The invention also relates to the use of the above detector in machines used in nuclear medicine, especially positron emission tomography scanners. According to another variant, the invention relates to the use of the above detector in detection machines for oil drilling.
Further details and characteristics will emerge from the description below of non-limiting preferred embodiments and from data obtained on specimens consisting of single crystals according to the invention.